Radiation curable adhesives containing acrylate functionality are known. However, this type of adhesive is not practical for flexible packaging applications because of odor associated with the presence of low molecular weight acrylates remaining in the crosslinked adhesive compositions, extractables and non-compliance with FDA regulations. In addition, radiation curable acrylate adhesives typically require a photoinitiator. These photoinitiator containing adhesives are unsuitable for use in flexible packaging applications such as the food packaging industry because of odor and non-compliance with FDA regulations related to unreacted initiator and initiator fragments. An example of this technology is U.S. Pat. No. 5,328,940 which discloses cationically cured compositions containing an epoxy compound, a hydroxy-terminated polyurethane and a photoinitiator. Although some photoinitiators are now accepted for indirect food contact (21 CFR 175.105) users are also concerned about unreacted photoinitiator fragments passing through films into the packaged material. Furthermore, photoinitiators have a propensity for imparting color to a coating or adhesive and adding substantially to the cost of the adhesive formulation.
European Patent 658,582 discloses a prepolymer composition containing at least two methacrylate and at least two urethane and/or urea groups for use in thermally cured adhesives. U.S. Pat. Nos. 5,859,084 and 5,741,829 disclose acrylic compositions that have a photoinitator covalently bonded to a polymer chain in order to minimize extractable fragments. Japanese unexamined patent no. 3-45216 discloses photocurable compositions comprised of an unsaturated urethane-acrylate having at least one (meth)acryloyl group and urethane linkage per molecule and an acetoacetyl group containing (meth)acrylate having both (meth)acryloyl groups and an acetate ester group per molecule. According to the above referenced Japanese patent, UV curing requires a photoinitiator. U.S. Pat. No. 3,925,349 claims and discloses a UV curable non-gelled Michael addition reaction product of an aliphatic amine, a stoichiometric excess of a polyester of acrylic acid and a photosensitizer to render the product sensitive to UV radiation. U.S. Pat. No. 4,284,731 discloses radiation curable compositions comprised of a urethane prepolymer prepared by reacting a polyol and an isocyanate, dicyclopentyloxyalkylacrylate and/or methacrylate, and a photoinitiator. U.S. Pat. No. 4,920,157 discloses radiation curable contact adhesives consisting of the reaction product of diisocyanate, a polyol, an alkanediol, a hydroxyalkyl (meth)acrylate, and a monohydroxylalkylcarboxylic acid. This reaction product is mixed with a monovinyl compound. According to the patent, if curing is to be accomplished by UV radiation a photoinitiator is used.
The current solventless adhesive technology that dominates the flexible packaging industry is a two component urethane adhesive. While strides have been made to make the adhesive safer by reducing the amount of monomeric isocyanate compounds, known as generation advancements in the industry, the curing reaction is still urethane based and typically takes hours to fully react. The time needed to reach full cure can range from 24 to 150 hours depending on how low of an extractable level is required for the application. Extractable levels required for most food packaging are as low as 5 to 50 parts per billion of aromatic amines.